This will be the first clinical study of the development of PHA-794428 in a pediatric population. Since differences in PK and/or PD response may occur between adult and pediatric subjects, it is deemed appropriate to first conduct an exploratory single dose study in pediatric patients to assess safety and tolerability in this patient population. In addition this will add pediatric data to facilitate the prediction of the optimal therapeutic dose to be tested in repeated dose phase 2b trials in children, using PK/PD modeling